linux-kernel - Re: floppy.c soft lockup

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Date:	Thu, 31 May 2007 10:28:39 -0400
From:	Mark Hounschell <markh@...pro.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>
Subject: Re: floppy.c soft lockup

Andrew Morton wrote:
> On Tue, 29 May 2007 13:31:05 -0400 Mark Hounschell <markh@...pro.net> wrote:
> 
>> Changes in floppy.c from 2.6.17 and 2.6.18 have broken an application I have. I have tracked 
>> it down to a single line of code. When the following patch is applied to the version in 2.6.18
>> my application works.
>>
>> --- linux-2.6.18/drivers/block/floppy.c 2006-09-19 23:42:06.000000000 -0400
>> +++ linux-2.6.18-crt/drivers/block/floppy.c     2007-05-29 09:12:20.000000000 -0400
>> @@ -893,7 +893,6 @@
>>                 set_current_state(TASK_RUNNING);
>>                 remove_wait_queue(&fdc_wait, &wait);
>>
>> -               flush_scheduled_work();
>>         }
>>         command_status = FD_COMMAND_NONE;
>>
>> I don't claim to understand the changes from 2.6.17 to 2.6.18 except for the devfs removal.
>> All I can say is this one line of code kills the application. I have tried to write a short pgm
>> that shows my problem but everything else I write seems to work. The application only runs
>> on SMP machines and uses process and irq affinities with real-time scheduling. When I turn
>> off process and irq affinities the application runs. 
>>
>> I have tried kernels up through 2.6.21.1 with the same results. All kernels from 2.6.18 up
>> require that I remove this one line of code or my application does not work?
> 
> Interesting.  I'd expect that the calling process is spinning, with realtime
> policy and is expecting some other process to do something (ie: run a workqueue).
> 
> If you keep the process and irq affinities, and disable the realtime policy
> does that also prevent the problem?
> 

Yes it does.

> It would be interesting it you could capture a few task traces while it is stuck:
> echo 1 > /proc/sys/kernel/sysrq then do ALT-SYSRQ-P a bunch of times and ALT-SYSRQ-T,
> see if you can work out where the CPU is stuck.
> 

I've attached the syslog output as a result of doing the above. I can't really make any kind of
determination from it myself as I don't really knowing what I'm looking at.

> ALso, 2.6.22-rc3 might have accidentally fixed this.
> 

No. Same thing there.  The traces attached are using 2.6.22-rc3.

Basically the main RT-process (which is a CPU bound process on processor-2) signals a
thread to do some I/O. That RT-thread (running on the other processor) does a simple 

ioctl(Q->DevSpec1, FDSETPRM, &medprm)

and there is no return from the call. That thread is hung.


Thanks 
Mark



View attachment "sysrq.txt" of type "text/plain" (25770 bytes)